(1) Field of the Invention
The present invention relates to an exhaust gas purifying system and a regeneration end determining method both suitable for use particularly in a diesel engine.
(2) Description of Related Art
Heretofore, there has been known a technique wherein an oxidation catalyst and a particulate filter (hereinafter referred to simply as “filter”) are disposed in an exhaust passage of a diesel engine, allowing a particulate matter (PM) contained in exhaust gas to be deposited on the filter, and the PM thus deposited on the filter is burnt to regenerate the filter continuously.
In the exhaust gas purifying system constructed as above, NO contained in exhaust gas is oxidized in the oxidation catalyst to produce NO2, then the NO2 and the PM on the filter are reacted with each other, allowing the PM to burn (be oxidized) to regenerate the filter continuously. NO2 exhibits a high function as an oxidizing agent in comparison with NO and permits the PM to be oxidized with a relatively low activation energy (that is, permits the PM to burn at a relatively low temperature).
In a certain operating condition of an engine there is a case where the exhaust gas temperature does not rise to an activation temperature of the oxidation catalyst, so that NO is not oxidized and hence a continuous regeneration of the filter is not effected. In such a case it is necessary to perform a forced regeneration different from the continuous regeneration.
As a method for the forced regeneration there is known a method wherein a heat source such as an electric heater is attached to the filter and an electric current is supplied to the heater, allowing PM to burn, or a method wherein fuel (HC) is fed to the oxidation catalyst and is allowed to undergo an oxidation reaction in the oxidation catalyst, the temperature of the filter is increased by the reaction heat to cause burning of the PM.
In the forced regeneration, however, the combustion temperature is higher than in the continuous regeneration and therefore it is necessary to positively control the state of burning of PM. More specifically, not only it is necessary to control the filter temperature with a high accuracy, but also it is necessary to accurately determine a filter regeneration start timing and a filter regeneration end timing.
As to an end timing of forced regeneration, it is generally determined on the basis of an elapsed time from the start of forced regeneration. Usually, an end of forced regeneration is determined upon lapse of a certain time from the start of forced regeneration (i.e., from the time-point of arrival of the filter temperature at a predetermined temperature as a PM burning temperature) (prior art 1).
Also in Japanese Examined Patent Application Publication No. H05(1993)-41809 (Patent Literature 1) there is disclosed a technique for determining an end timing of forced regeneration. According to the technique disclosed in Patent Literature 1, a correction coefficient k is set in accordance with a filter inlet temperature Tin and there is calculated the product (k·Δt) of the coefficient k and the time Δt during which the filter holds the inlet temperature Tin, further, an integrated value of the product, Σ(k·Δt), is determined and a forced regeneration is stopped when the integrated value Σ(k·Δt) reaches a predetermined value or a larger value (prior art 2).
However, the prior art 1 involves a problem that it is impossible to determine an end of forced regeneration correctly. More particularly, the flow rate of exhaust gas varies depending on the state of vehicular running and an operating condition of an engine (that is, the flow rate of exhaust gas is not constant), so in a forced regeneration, the amount of PM which burns in unit time differs depending on a vehicular running condition and an operating condition of an engine. Thus, in a method which controls the determination of a forced regeneration end on the basis of the regeneration time, it is impossible to make such a determination of a forced regeneration end as is suitable for a vehicular running condition and an operating condition of an engine. If the forced regeneration time is too long, the fuel economy is deteriorated, while if it is too short, the regeneration of filter will not be effected to a satisfactory extent.
In the technique (prior art 2) disclosed in Patent Literature 1, the time is multiplied by the coefficient k which is proportional to the filter inlet temperature Tin. However, it is not that the combustion efficiency of PM depends solely on temperature. Thus, even with this technique, it is impossible to make a correct determination of a regeneration end. Although in this technique the time is multiplied by the correction coefficient k, an end timing is determined basically depending on the regeneration time Δt and thus a regeneration end cannot be determined accurately.